Mechanical and tribological properties of sputter deposited nanostructured Cr–WS2 solid lubricant coatings

Solid lubricant coatings of WS 2 and Cr–WS 2 (15–50 at.% Cr) prepared using an unbalanced magnetron sputtering system were evaluated for their mechanical and tribological properties. Nanoindentation results indicated that addition of Cr helped in improving the mechanical properties and the elastic recovery ability of Cr–WS 2 coatings. The adhesive strengths of Cr–WS 2 coatings were evaluated using a nanoscratch tester and from the nanoscratch profiles, critical load values and optical images, it was evident that the adhesion of Cr–WS 2 coatings increased with an increase in the Cr content. Further analysis of the nanoscratch data indicated that WS 2 coatings exhibited large amount of plastic deformation compared to Cr–WS 2 coatings which showed a combination of elastic–plastic deformation. However, micro-tribometer measurements at a load of 2 N showed that the tribological properties of Cr–WS 2 coatings deteriorated with an increase in the Cr content. For example, Cr–WS 2 coatings prepared at Cr content ≥ 33 at.% failed after a sliding distance of 1 m. On the other hand, WS 2 and Cr–WS 2 coatings prepared at low Cr contents (15–23 at.% Cr) exhibited a stable friction coefficient (50–60% relative humidity) in the range of 0.10–0.13 for a sliding distance of 14 m. Micro-Raman spectroscopy data of the worn films taken after a sliding distance of 14 m indicated the presence of WS 2 transfer films for WS 2 and Cr–WS 2 coatings prepared at low Cr contents. For Cr–WS 2 coatings with Cr content ≥ 33 at.%, the worn films consisted predominantly of WO 3 . After an extended sliding distance of 50 m, Cr–WS 2 coatings (15–23 at.% Cr) outperformed WS 2 coating which failed after 20 m. Further, the coatings prepared at low Cr contents did not show any failure even after a sliding distance of 200 m. At a higher load of 7 N, Cr–WS 2 coating with 15 at.% Cr exhibited the best performance with a friction coefficient of 0.07 up to a sliding distance of 72 m. These results indicate that the amount of Cr in the WS 2 matrix needs to be controlled judiciously to obtain improved mechanical and tribological properties in Cr–WS 2 solid lubricant coatings.